Next-generation antithrombotics in ischemic stroke: preclinical perspective on 'bleeding-free antithrombosis'

Abstract

The present antithrombotic drugs used to treat or prevent ischemic stroke have significant limitations: either they show only moderate efficacy (platelet inhibitors), or they significantly increase the risk for hemorrhages (thrombolytics, anticoagulants). Although most strokes are caused by thrombotic or embolic vessel occlusions, the pathophysiological role of platelets and coagulation is largely unclear. The introduction of novel transgenic mouse models and specific coagulation inhibitors facilitated a detailed analysis of molecular pathways mediating thrombus formation in models of acute ischemic stroke. Prevention of early platelet adhesion to the damaged vessel wall by blocking platelet surface receptors glycoprotein Ib alpha (GPIbα) or glycoprotein VI (GPVI) protects from stroke without provoking bleeding complications. In addition, downstream signaling of GPIbα and GPVI has a key role in platelet calcium homeostasis and activation. Finally, the intrinsic coagulation cascade, activated by coagulation factor XII (FXII), has only recently been identified as another important mediator of thrombosis in cerebrovascular disease, thereby disproving established concepts. This review summarizes the latest insights into the pathophysiology of thrombus formation in the ischemic brain. Potential clinical merits of novel platelet inhibitors and anticoagulants as powerful and safe tools to combat ischemic stroke are discussed.

Figure 1

rHA-Infestin-4 protects from ischemic stroke in mice. rHA-Infestin-4 is a Kazal-type serine protease inhibitor originally isolated from the gut of the sanguivorous kissing bug Triatoma infestans (upper panel) that inhibits FXIIa with a high level of specificity. Pretreatment of mice with rHA-Infestin-4 dramatically reduced infarct volumes on day 1 after 60 minutes of tMCAO, as shown by infarct volumetry from 2,3,4-triphenlytetrazoliumchloride (TTC)-stained brain slices (lower panels). The ischemic areas appear in white. Importantly, no signs of increased intracranial bleeding were found on day 1 after rHA-Infestin-4 treatment (adapted from Hagedorn et al, 2010). Ctrl, Control; Inf-4, rHA-Infestin-4; FXIIa, factor IIa.

Figure 2

Distinct steps of platelet adhesion, activation, and aggregation at the activated endothelium. (A) The initial adhesion of platelets (tethering) is mediated by the binding of the glycoprotein (GP)Ib-V–IX receptor complex to the A1 domain of the von Willebrand factor (VWF) on endothelial cells. Additionally, binding to P-Selectin can enhance platelet recruitment to the intact vessel wall. (B) In a second step, interactions between GPVI and collagen stabilize the thrombus. Moreover, it comes to a cellular activation with secretion of platelet agonists (e.g., adenosine diphosphate, ADP) and transformation of the GPIIb/IIIa receptors to a state with high affinity. (C) The common final pathway of the platelet activation via the GPIIb/GPIIIa (integrin–fibrinogen) pathway culminates in an irreversible platelet aggregation and subsequent thrombus growth.

Figure 3

Simplified diagram of Ca²⁺ homeostasis in platelets. On activation of several receptors by ligands such as adenosine diphosphate (ADP), von Willebrand factor (VWF) or collagen, different phospholipase (PL)C isoforms hydrolyze phosphatidilinositol-4,5-bisphosphate (PIP₂) to inositol-1,4,5-trisphosphate (IP₃) and diacyl-glycerol (DAG). Inositol-1,4,5-trisphosphate leads to a Ca²⁺ release from intracellular stores (endoplasmic reticulum, ER). On Ca²⁺ release STIM1 activates the Ca²⁺ channel Orai1 in the plasma membrane that mediates Ca²⁺ influx from the extracellular space. Sarcoplasmic/endoplasmic reticulum Ca²⁺ ATPases (SERCAs) are involved in the counteracting mechanisms (adapted from Varga-Szabo et al, 2009). IP₃R, IP₃-receptor; STIM1, stromal interaction molecule 1.